This invention relates to a process for preparing paper consisting essentially of a para-aromatic polyamide (referred to hereinafter as para-aramid in some cases). The para-aramid paper obtained by the process of this invention is useful particularly as insulating paper for composite materials having a honeycomb structure and the like in application fields requiring heat resistance and high strength.
Para-aramid fibres have excellent properties, such as high strength, high stiffness, and high heat resistance, and para-aramid pulp prepared from para-aramid fibres has been widely used as a substitute for asbestos. However, said para-aramid fibres do not melt and hence para-aramid paper prepared from said para-aramid pulp has no agglutinate portions (called entangled portions in some cases). Consequently, para-aramid paper has low strength and so is difficult to use as insulating paper and the like.
Extensive research was performed to solve the above problems. For example, Japanese Patent Application Kokai No. 59/163 418 discloses that the agglutinate portions between fibrils can be improved by using pulp obtained by fibrillating fibres consisting of para-aramid and aliphatic polyamide.
In Japanese Patent Application Kokoku No. 3/39 539, a sheet-like paper comprising para-aramid short fibres is also proposed for improving the heat resistance and strength of a paper composed of meta-aromatic polyamide (referred to hereinafter as meta-aramid in some cases). Extremely fine short fibres (called fibrils in some cases) are obtained by violently agitating a meta-aramid solution with high shearing in a non-solvent (usually an aqueous medium). Said fibrils agglutinate upon drying, and as a result meta-aramid paper comprising as an essential component fibrils composed of meta-aramid has a high strength. Since para-aramid fibres have a higher fibre strength than meta-aramid fibres, combining para-aramid short fibres with fibrils composed of meta-aramid will give paper having a higher strength and a higher heat resistance.
The above-mentioned system is one in which a component other than para-aramid is added in order to form agglutinate portions in para-aramid paper, and hence said other component adversely affects the high stiffness and high heat resistance characteristic of para-aramid fibres.
Japanese patent application Kokai No. 3/14 832 discloses a method for preparing para-aramid paper using as a binder a para-aramid fibrous gel composition consisting of para-aramid, an amide solvent, an alkaline earth metal, and N-methylpyrrolidine. However, the Examples thereof show that the breaking length is 0.34 lb/in/oz/yd2 (corresponding to 0.18 km), which is only about twice as high as the breaking length of paper prepared from commercially available para-aramid pulp.